Lasers have been used in many applications for a number of years. Laser manufacturers have continually sought to improve beam quality and operating efficiency of their lasers in order to provide lasers that are capable of providing precise, high-energy beams at reasonable operating costs.
The computer chip industry illustrates how improvements in beam quality and efficiency of operation have been utilized. In one application, a photoresist is spun onto a substrate such as a silicon wafer and is subsequently patterned in a clean room by exposing selected areas of the photoresist to ultraviolet (UV) light. The patterned photoresist defines structures in the silicon wafer that eventually become semiconductor devices. If these structures can be defined more precisely, the structures can be made smaller and with less space between them, and, consequently, more structures or semiconductor devices can be fit into a small area. Lasers have been used to provide the UV light that patterns photoresist. A portion of the photoresist on the wafer is patterned using a burst of laser light, the wafer is quickly repositioned, and another portion of the photoresist is patterned using another burst of laser light. This process is repeated until the entire wafer has been patterned. As beam parameters such as divergence, power, and directional control are improved, semiconductor devices can be made smaller and faster, providing chips of greater operational capacity and higher speed but equivalent physical size to previous, less-powerful chips made using older lasers. Higher production speed and improved efficiency in utilizing power and consumable resources such as filtered cooling air and water reduces production costs, helping to keep computer chip prices reasonable, but also requires that the laser provide more bursts of light in a unit of time while maintaining a focused beam with little divergence and directional shift.
The medical profession has also shown a continuing need for lasers having beams of low divergence, high power, and precise directional control. For example, corneal or retinal incisions must be made accurately and quickly with minimal to no damage of surrounding tissue. Beams having improved control over their power allow quick incisions to be made to the correct depth, minimizing the time of exposure of tissue to laser light and minimizing the chances that stray incisions will occur because of equipment vibration, for example.
One laser that has been particularly useful in photolithography and in medical applications is the gas discharge laser, particularly the excimer laser, due to the highly-energetic photons emitted in the ultraviolet range of the electromagnetic spectrum. The beam from the excimer laser has sufficient energy to break chemical bonds in organic material without raising the temperature of the surrounding material. The excimer laser's ability to perform this cold photochemical action allows the system to produce laser light of the correct frequency and power to be used in photolithography or in medical applications.
One commercial excimer laser 100 is illustrated in FIG. 1. An enclosure 110 houses much or all of the equipment associated with producing laser light. A commercial excimer laser uses such equipment as a discharge chamber 120 that generates the beam, a heat exchange system (a portion of which is shown at 120 in FIG. 1 or at 250 in FIG. 2) to add heat to or remove heat from the discharge chamber, a high-voltage power supply 130, a pulse power module 140, control circuitry 150, a laser gas management system that optionally includes a vacuum pump 160, and ventilation equipment such as a ventilation stack 170 that is attached to a vent or vacuum duct in the room in which the laser is located. The enclosure typically has multiple openings 180 located on multiple sides of the enclosure through which air outside the enclosure is drawn inside, but little attention has been given to utilizing the air drawn into the enclosure in an efficient manner. Many times, the gas used to produce the laser beam contains biologically-incompatible gases such as fluorine (F.sub.2), so conventional thinking has been to draw large quantities of air into the enclosure and to provide multiple pathways for air to flow in an attempt to clear the enclosure of any of these gases that might be leaking from the laser discharge chamber within the enclosure. However, the increasing complexity and precision of chip manufacturing has required the air surrounding silicon wafers and therefore surrounding the lasers used to process those wafers to be essentially free of any dirt or contaminants. This air is highly filtered and conditioned to clean-room specifications and is therefore extremely expensive. It is one object of this invention to provide a laser mounted in an enclosure and having reduced consumption of this expensive air that meets clean-room standards.
The laser beam in a commercial gas laser is generated in a discharge chamber such as a discharge tube, a box-like housing, or other suitably-shaped chamber that is usually located in the enclosure. A heat exchange system is used to maintain the discharge chamber at an approximately constant temperature.
To produce the laser beam in a gas laser, gas within the discharge chamber is subjected to sufficient energy to pump electrons of atoms or molecules to a higher or excited energy state. The energetic atoms or molecules are then stimulated to emit a photon by external photons. The light generated by this stimulated emission is reflected between two mirrors, causing further stimulated emission of photons. As more photons are emitted, the power of the light beam is amplified, and when the beam reaches a threshold level of power, part of the beam passes through one of the mirrors, which is only partially reflective.
The pressure and temperature of the gas at lasing conditions are critical parameters that affect efficiency of the laser and quality of the laser beam. One method of controlling the pressure of the gas while also controlling the temperature is to provide a volume of gas that is essentially fixed by the volume of the laser chamber. The efficiency of the laser is highest at a gas temperature selected for the particular lasing gas, normally between 35 and 55.degree. C. The laser chamber must maintain gas temperature to better than a few degrees Centigrade for constant power output. The laser-beam's size, divergence, power, and direction are also affected by gas temperature. If the temperature of the gas is not maintained sufficiently constant, the beam can become too large, diluting the power of the beam. The beam can diverge rather than converge on the target, further diluting its power. Additionally, the beam can travel in an undesired direction, either hitting a target at an incorrect location or missing the target entirely if the temperature is not well-regulated. Careful control of gas temperature can therefore provide efficient operation of and high-quality light from a laser.
In a commercial laser, especially one operating at high powers, such as 5 W or greater, precise control of the temperature of the lasing gas is very difficult to achieve. The lasing gas is instantaneously subjected to a large quantity of energy to create excited molecules and/or atoms. For example, in a broadband krypton fluoride excimer laser, approximately 3% of the energy is converted into laser light, and much of the unused energy generates heat. Consequently, a large quantity of heat is generated in a very short period of time. The resultant fast increase in gas temperature must be sensed quickly, and heat must be removed rapidly in order to maintain the temperature of the gas constant. Gas temperature also drops rapidly when the power used to generate the beam is shut off, so again the temperature change must be sensed quickly, and the rate at which heat is removed must be adjusted rapidly to maintain the gas temperature constant. Further, many industrial lasers are operated in a burst mode, wherein the beam is generated for a short period of time, followed by a short idle period while the beam's target is repositioned prior to its next burst. For example, a laser operating in burst mode may generate a beam for approximately one to a few seconds, then idle for a short period such as 0.1 to 3 seconds, at which time the cycle repeats. Thus, much of the time, a laser is in a transitory, non-equilibrium state in which control of gas temperature is very difficult.
There have been a number of systems devised to control the pressure of lasing gas. U.S. Pat. No. 5,117,435 discloses a pressure regulating system for a gas laser, wherein a thermocouple mounted to the stem of an anode that is used to provide energy to pump electrons into a more energetic state measures the temperature of the anode and controls the temperature of the anode and, consequently, the pressure in the laser, by opening a solenoid valve to admit more gas to the laser chamber. The temperature of the cooling water, which affects the temperature read by the stem-mounted thermocouple, is used to normalize the stem-mounted thermocouple's signal to compensate for the cooling water's effect on the signal from the stem-mounted thermocouple. The heat exchanger establishes the temperature of the gases within the laser, and the pressure of those gases is maintained by adding new gas in response to the anode temperature. The patent is silent on how or whether the water flowing through the heat exchanger is regulated.
Another system uses a thermocouple mounted onto the discharge chamber wall to sense changes in the wall temperature caused by heating or cooling of the gases within the chamber. Thermocouples have traditionally been located out of the presence of "pumped" hot lasing gases, especially where reactive gases such as fluorine are present, in order to prevent consumption of the reactive gas and contamination of the lasing gas with metal fluorides. The signal from the wall-mounted thermocouple is converted to a voltage that is used to open a solenoid-controlled on/off water valve for a specified period of time. The temperature of the lasing gas fluctuates because of the large lag between the time that the gas temperature increases and the time that the thermocouple senses the chamber wall temperature increase caused by the increased gas temperature. Further, the temperature of the lasing gas fluctuates approximately sinusoidally because the on/off valve provides either full flow of cooling water or no flow of cooling water.
U.S. Pat. Nos. 4,760,583, 4,547,885, 4,661,958, 4,707,837, and 4,502,145 disclose a system for maintaining gas within a support tube supporting the mirrors and enveloping the laser discharge tube at a constant temperature, so that the support tube maintains proper alignment of the laser discharge tube and mirrors. These patents also disclose a separate system to maintain gas pressure constant to provide a laser beam of consistent quality. Temperature of the gas is measured by a thermocouple located outside the laser chamber. The thermocouple controls a solenoid on/off water valve for the heat exchanger, and cooling of the gas is controlled by varying the length of time that the water valve is opened. A portion of the gas is continually drawn out of the laser, and a pressure sensor regulates the rate at which fresh gas is introduced so that the gas pressure remains essentially constant. Temperature fluctuations within the laser itself are not sensed and therefore there is no compensation for fluctuations, since the thermocouple is located outside the laser and since the gas discharged from the laser passes through two heat exchangers before contacting the heat exchanger.
U.S. Pat. No. 5,084,885 uses the temperature of gas discharged from a gas laser and sensed at the inlet of a blower to prevent the blower from being damaged by overheating. The discharged gas is passed through a heat exchanger before it contacts the thermocouple so that heat is removed prior to the gas contacting the blower. Other patents similarly use a temperature measurement taken on or in the laser chamber for other purposes. U.S. Pat. No. 4,573,159 uses a plurality of thermocouples mounted on laser tube mounts or support plates to control an equal plurality of fans to maintain each mount at a constant temperature to maintain proper discharge tube and mirror alignment. In U.S. Pat. No. 5,091,914, a thermocouple is used to control the ambient air temperature around a laser discharge tube when the laser is idling in order to maintain proper alignment of the mirrors around the discharge tube. U.S. Pat. No. 5,005,929 provides a way to assure a laser beam is accurately positioned by comparing the temperature near a scanner position sensor to the ambient temperature and adjusting a positionable mirror.
It is an object of the invention to provide a laser wherein the gas temperature in the discharge chamber is quickly measured to provide a laser wherein the gas temperature is quickly and accurately controlled. These and other objects and advantages are apparent from the disclosure herein.